Natural gas hydrates are also called “combustible ice”. The “cage compound” formed by methane-based hydrocarbon gas and water under certain temperature and pressure conditions is of a white crystalline structure, and has a carbon content equivalent to twice total reserves of world-wide known energy sources, such as coal, oil and natural gas. Therefore, natural gas hydrates, especially marine natural gas hydrates, are generally considered to be a novel clean energy source that will replace coal, oil and natural gas in the 21st century, and are also a new energy source with large reserves that has not been developed yet at present.
According to whether or not a skeleton structure of an ore bed in which hydrates have been decomposed and gasified can be maintained without loosening and falling (i.e., load-bearing), seabed natural gas hydrate ore beds can be divided into diagenetic ore beds and non-diagenetic ore beds. At present, the mainstream opinion is that: diagenetic hydrates are more likely to be mined in the technical level than non-diagenetic hydrates, but the vast majority of seabed hydrates are non-diagenetic.
At present, main methods considered at home and abroad for hydrate mining include a heat injection method, a pressure reduction method, a carbon dioxide replacement method, a chemical reagent injection method, and the like. These mining methods ask for the requirements that an upper layer of hydrates has a good capping layer with a large thickness and a solid structure and the skeleton of the ore bed in which hydrates have been mined and decomposed can be still maintained without loosening, i.e., the ore bed is a diagenetic hydrate ore bed itself, otherwise, after gases are decomposed from the hydrates, the skeleton structure of the ore bed will disappear, and the large amount of gases produced by decomposition will change the formation pressure. In addition, the above-mentioned mining methods cannot effectively control the decomposition rate of hydrates and the spatial decomposition range of the ore bed, which may cause geological and environmental disasters, because the formation of hydrate decomposition chain reactions will cause major disasters. Another risk is that, after the hydrates are decomposed and gasified, if the capping layer is not good, gases may diffuse through the capping layer. To sum up, the above-mentioned mining methods have still not been able to effectively solve the above problems and are no longer expected to be in commercial mining.
In view of natural gas hydrates on the surface of the deep sea, some scholars have proposed a “solid-state fluidization” mining method. In this method, in the case of not actively changing the temperature and pressure of a seabed hydrate ore bed, that is, avoiding the occurrence of decomposition of hydrates and the resulting environmental and geological disasters, natural gas hydrates are directly broken into solid particles, and the mixture of the natural gas hydrate particles and sea water is pumped to the sea surface through an airtight pipeline, and then separated, decomposed and gasified.
Solid-state fluidization provides a new idea for the mining of shallow layer non-diagenetic natural gas hydrates of the deep sea. At present, a mining device for seabed shallow layer hydrates is a self-propelled mining vehicle, but it is not suitable for seabed shallow layer hydrates having certain burial depth and is low in economical efficiency.